1. Field of the Invention
The present invention relates generally to an optical fiber, and more particularly to low-loss, silica-germania-containing optical fiber doped with phosphorous and alkali metal oxides.
2. Technical Background
Attenuation is a principal limiting attribute of optical fibers. Optical fiber loss, for example, plays an important role in limiting the distance between optical fiber amplifiers. This is particularly important in long distance and ultra-long distance networks such as, for example, undersea applications, where such amplifiers represent a significant system cost, as well as a major factor in system reliability. Consequently there is tremendous commercial interest in reducing attenuation to the lowest possible level.
Electromagnetic radiation traveling through an optical waveguide fiber is subject to attenuation or loss due to several mechanisms. Although some of these mechanisms can not be reduced, others have been eliminated, or at least substantially reduced.
A particularly problematic component of optical fiber attenuation is the attenuation due to absorption by the optical waveguide fiber of impurities present in the light guiding region of the fiber. Particularly troublesome is the attenuation caused by the hydroxyl radical (OH), which can be formed in the optical waveguide fiber when a source of hydrogen is present in the fiber material, or when hydrogen available from several sources during the fiber manufacturing process diffuses into the glass. Silica bodies of the type used in optical fiber and optical fiber preform manufacture can contain a substantial amount of OH. Generally speaking, the hydrogen bonds with the oxygen available in the SiO2 and/or GeO2 and/or other oxygen containing compound in the glass matrix to form the OH and/or OH2 bonds referred to generally as “water”. The attenuation increase due to OH or water in the glass can be as high as about 0.5 to 1.0 dB/km.
Silica glass doped with an alkali metal oxide, such as K2O, has been shown to be capable of reducing attenuation in optical fibers. Nevertheless, optical fiber doped with alkali metal oxides can result in unacceptable amounts of hydrogen induced attenuation change.